1. Field of the Invention
The present invention relates to an apparatus that generates gamma voltage used in a display device, and more particularly, to an apparatus and method for generating gamma voltage in a display device.
2. Discussion of the Related Art
Recently, various flat display panel technologies have become more common due to reduced weight and bulk in comparison to cathode ray tube (CRT) technology. Such flat display panel technologies include liquid crystal displays, field emission displays, plasma display panels, and electro-luminescence (EL) display devices. Among these, the EL display device is a self-luminous device that causes a fluorescent substance to emit light by a re-combination of an electron and a hole, and can be generally classified into an inorganic EL where an inorganic compound is used as the fluorescent substance and an organic EL where an organic compound is used. The EL display device has many advantages such as low driving voltage self-luminescence, thin profile, wide-viewing angle, rapid response speed, and high contrast. Hence, the EL device is expected to be a next generation display device.
The organic EL device generally includes an electron injection layer, an electron transport layer, a light-emitting layer, a hole transport layer, and a hole injection layer. These elements are deposited between a cathode and an anode. In such an organic EL device, when a specified voltage is applied between the anode and the cathode, an electron generated from the cathode moves to the light-emitting layer through the electron injection layer and the electron transport layer. Meanwhile, a hole generated from the anode moves to the light-emitting layer through the hole injection layer and the hole transport layer. Accordingly, the re-combination of the electron and the hole supplied from the electron transport layer and the hole transport layer causes light to be emitted in the light-emitting layer.
An active matrix EL display device using such an organic EL device, as shown in FIG. 1, includes an EL panel 20 having pixels 28 each arranged at an area defined by a scan line SL and a data line DL crossing each other, a scan driver 22 driving the scan lines SL of the EL panel 20, a data driver 24 driving the data lines DL of the EL panel 20, and a gamma voltage generator 26 applying a plurality of gamma voltages to the data driver 24. The scan driver 22 applies scan pulses to the scan lines SL to sequentially drive the scan lines SL. The data driver 24 converts a digital data signal input from the outside into an analog data signal based on the gamma voltage from the gamma voltage generator 26. The data driver 24 also applies the analog data signal to the data lines DL whenever the scan pulse is applied. Each pixel 28 receives the data signal from the data line DL to generate a light corresponding to the data signal when the scan line SL is supplied with the scan pulse.
To this end, each pixel PE, as shown in FIG. 2, includes an EL cell OEL having a cathode connected to a ground voltage source GND, and a cell driver 30 connected to the scan line SL, the data line DL, a supply voltage source VDD, and an anode of the EL cell OEL for driving the EL cell OEL. The cell driver 30 includes a switching thin film transistor T1 with a gate terminal connected to the scan line SL, a source terminal connected to the data line DL, and a drain terminal connected to a first node N1, a driving thin film transistor T2 with its gate terminal connected to the first node N1, a source terminal connected to the supply voltage source VDD, and a drain terminal connected to the EL cell OEL, and a capacitor C connected between the supply voltage source VDD and the first node N1.
The switching thin film transistor T1 is turned on to apply the data signal from the data line DL to the first node N1 if the scan line SL is supplied with the scan pulse. Having been applied to the first node N1 the data signal charges the capacitor C and, the same time, is applied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 controls the amount of current I applied to the EL cell OEL from the supply voltage source VDD in response to the data signal applied to the gate terminal, thereby controlling the amount of light-emission of the EL cell OEL. Because the data signal is held from the capacitor C even after the switching thin film transistor T1 is turned off, the driving thin film transistor T2 applies the current I from the supply voltage source VDD to the EL cell OEL until the data signal of the next frame is applied, thereby causing the light-emission of the EL cell OEL to be sustained.
In such a manner, the related art EL display device applies a current signal proportional to an input data to each of EL cells OEL, and the EL cells OEL emit light to display a picture. And, the EL cells OEL include an R cell OEL having a red fluorescent substance (hereinafter, R), a G cell OEL having a green fluorescent substance (hereinafter, G), and a B cell OEL having a blue fluorescent substance (hereinafter, B) to realize color. The three cells OEL R, G, B are then mixed to realize a color for a pixel.
FIG. 3 illustrates a detailed circuit configuration of the gamma voltage generator 26 shown in FIG. 1. The gamma voltage generator 26 shown in FIG. 3 generates a gamma voltage set having a number n of gamma voltages GMA1 to GMAn with different voltage values than one another corresponding to different brightness levels than one another. In the example of FIG. 3, the number n is five. To this end, the gamma voltage generator 26 has a number (n+1) of resistors R1 to Rn+1 connected in series between the supply line of the supply voltage VDD and the supply line of the ground voltage GND. The gamma voltages GMA1 to GMAn with different voltage values from one another are generated in each of voltage division points of the (n+1) number of the resistors R1 to Rn+1.
In this way, the gamma voltage generator 26 of the related art generates the gamma voltage set having the n gamma voltages GMA1 to GMAn, and the data driver 24 converts the digital data into the analog data signal based on the gamma voltage set, thereby controlling the current signal applied to the EL cell OEL. Accordingly, the gamma voltage set generated from the gamma voltage generator 26 influences the brightness of the EL display device. However, there arises a necessity for a scheme to adaptively control brightness in accordance with the brightness of an outside environment to provide a clear picture regardless of place or conditions.